Rapid thermal processing lamp and method for maufacturing the same

ABSTRACT

A method and system for inductively coupling energy to a heating filament ( 7 A′,  7 B′,  7 C′,  7 A,  7 B,  7 C) in a thermal processing environment. By applying AC power to a coil antenna ( 11 ) and inductive coupling to a filament (e.g., a halogen lamp filament), a number of connections that are subject to fatigue is reduced, thereby increasing the reliability of the heater ( 2 A,  2 B). Such an environment can be used to process semiconductor wafers ( 3 ) and liquid crystal displays.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention is related to the following patentapplications: “Multi-Zone Resistance Heater,” U.S. Provisional SerialNo. 60/156,595 filed Sep. 29, 1999; and Attorney Docket No. 197159WO,entitled “Multi-Zone Resistance Heater,”filed Sep. 18, 2000. Thecontents of each of those applications is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is directed to an apparatus for rapidlyheating a substrate to be processed, and more particularly to anapparatus for heating during a rapid thermal processing of asemiconductor wafer or a liquid crystal display.

[0004] 2. Discussion of the Background

[0005] Rapid Thermal Processing (RTP) and epitaxy in semiconductor chipmanufacturing are processes where a heating method (e.g., an opticalheating method) is used to ramp up the temperature of a semiconductorwafer rapidly, hold at a steady state high temperature for a period oftime, and then ramp down rapidly. RTP allows the wafer to be heated veryquickly to its activation temperatures (e.g., at least 1000 degrees C.).The activation temperature is the temperature at which a correspondingprocessing step (e.g., deposition, implantation, diffusion, removal orformation of key materials) is stabilized. The temperature and theperiod of time at which that temperature is maintained must be executedprecisely for each processing step. Overheating can cause dopants topermeate subjacent layers, and under-heating can produce layers withuncontrolled characteristics.

[0006] RTP in semiconductor processing has been used for annealing ofsemiconductor wafers. One such application of RTP is to anneal the waferafter ion implantation. The RTP heaters used for these purposes musthave a high uniformity; that is, the temperature must be uniform acrossthe entire wafer surface. In most known RTP heaters, the solid angle ofthe filament—which is the solid angle subtended by the filament whenviewed from the wafer surface—is extremely small, on the order of atenth of a radian. The view factor area then is defined as the areaprojected on the wafer by the solid angle of the filament (i.e., thesolid angle of the filament multiplied by the distance between thefilament and the wafer). When the total solid angle for the total wafercoverage is 4π radians, a solid angle of a tenth of a radian covers onlyabout 1% of the wafer. Reflectors outside a container wall of quartzonly extend the radiation field for the short wavelength radiationfield.

[0007] Known RTP heaters have a relatively short lifetime because thoselamps have a physical connection between their filaments and their powersupplies. Such lamps are sealed on the ends, and the wires connectingthe filaments are brought to the outside world to be physicallyconnected to a power supply. This physical connection causes a highfailure rate due to the filament input/output connection failure, whichin turn is due to the stresses built up at the interface as a result ofthe different thermal expansion rates of the metals in the connection,during normal lamp operation. Known tungsten halogen lamps are formedinto nested group of rings acting as an RTP heater. One such embodimentincludes smaller rings toward the center and larger rings toward outsideedge.

[0008] Additional information about RTP systems can be found in: (1)U.S. Pat. No. 5,715,361 entitled “Rapid thermal processinghigh-performance multi-zone illuminator for wafer backside heating”,issued to CVC Products, Inc., Fremont, Calif., on Feb. 3, 1998; (2) U.S.Pat. No. 4,857,689, entitled “Rapid thermal furnace for semiconductorprocessing”, issued to High Temperature Engineering Corporation,Danvers, Mass. on Aug. 15, 1989; (3) U.S. Pat. No. 5,504,831, entitled“System for compensating against wafer edge heat loss in rapid thermalprocessing”, issued to Micron Semiconductor, Inc., Boise, Id. on Apr. 2,1996; (4) U.S. Pat. No. 5,719,991, entitled “System for compensatingagainst wafer edge heat loss in rapid thermal processing”, issued toMicron Technology, Inc., Boise, Id. on Feb. 17, 1998; (5) U.S. Pat. No.5,601,366, entitled “Method for temperature measurement in rapid thermalprocess systems”, issued to Texas Instruments Incorporated, Dallas, Tex.on Feb. 11, 1997; and (6) U.S. Pat. No. 5,881,208, entitled “Heater andtemperature sensor array for rapid thermal processing thermal core”,issued to Sematech, Inc., Austin, Tex. on Mar. 9, 1999.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide an RTP lampwith a lower failure rate and/or cost than known lamps.

[0010] It is a further object of the present invention to increase theview factor area, or the solid angle of the filaments, of an RTP lamp sothe radiation field can cover a large area of the wafer, with anincreased uniformity of the radiation field. In one such embodiment, thepresent invention preferably forms lamp elements into the shape of largearea rings. Moreover, the larger the filament is, the larger the solidangle is, so the larger the view factor area is and the higher theuniformity of the radiation field is. For systems having a shorterdistance from the heat source to the wafer, this solid angle becomesincreasingly important.

[0011] It is yet a further object of the present invention to increase alamp emitting area, so that the radiated power (i.e., the product of theemitting area and the temperature of the emitting area) is increased. Inone such embodiment, the present invention preferably includes anadjustable distribution of power with high uniformity.

[0012] It is another object of the present invention to provide a lampincluding filaments that are not physically connected to the powersupply, to other filaments or to the outside world. To achieve thisobject, the present invention preferably inductively powers eachindividual filament (e.g., a tungsten ring) that is sealed inside itsown cavity. Preferably at least one fluid cooled conduction cup is usedto isolate the induction coils. Preferably the heating of the inductioncoils is reduced by utilizing a gold-plated plate with multiple slotswhich allow the RF field to leak out.

[0013] It is an additional object of the present invention to provide avacuum window in the processing chamber. To achieve this object, thepresent invention preferably uses the upper quartz plate of the lamp asa vacuum window since it is sufficiently rigid to support the vacuum andyet thin enough to pass a very high intensity light.

[0014] It is a further object of the present invention to integrate thelamp into an upper injection plate for fast processing (e.g., heating ofwafer to 1000 DEG C in less than 5 seconds and with uniformity less than±3 DEG C.).

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] A more complete appreciation of the invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

[0016]FIG. 1 is a cross-sectional view of a heater according to a firstembodiment of the present invention;

[0017]FIG. 2 is a top view of plural heating areas according to oneaspect of the present invention;

[0018]FIG. 3 is a side view of a stacked heater according to the presentinvention;

[0019]FIG. 4 is a conceptual diagram of an angle of illumination for anarea according to the present invention; and

[0020]FIG. 5 is a partial top view of a heater according to a secondembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] Referring now to the drawings, wherein like reference numeralsdesignate identical or corresponding parts throughout the several views,FIG. 1 illustrates a first embodiment of the present invention. Theapparatus includes a chamber 1 to which heated lamp assemblies 2 a and 2b (e.g., RF induction heated lamp assemblies) are fitted. A substrate 3(e.g., a silicon wafer or LCD panel) to be heat-treated is locatedsubstantially midway between the two lamp assemblies (2 a and 2 b) onpins 4 (e.g., quartz pins). (As would be apparent to one skilled in theart, the present invention may selectively and alternately illuminate(1) the top side, (2) the bottom side or (3) both sides of the substrate3.) Alternately, a single lamp assembly can be used. The lamp assemblies2 a and 2 b include a first isolation plate 5, a second isolation plate6, and at least one of (1) filament rings 7 a, 7 b, 7 c, 7 a′, 7 b′ and7 c′, (2) a ring of filament coils, (3) a wavy filament structure, or(4) a ring made of weaving wires. The use of a coil or wavy structuremay decrease the build-up of stresses in the ring, leading to increasedlife expectancy. Filaments can be fabricated from any or a combinationof: tungsten, tungsten alloy, platinum, Kanthal, Nikrothal andAlkrothal, which are registered trademark names for metal alloysproduced by Kanthal Corporation of Bethel, Conn. The Kanthal familyincludes ferritic alloys (FeCrAl) and the Nikrothal family includesaustenitic alloys (NiCr, NiCrFe); however, tungsten is preferred for thefilament.

[0022] In a preferred embodiment, the first and second isolation plates(5 and 6) are formed from quartz. Such isolation plates may also bereferred to hereinafter as a substrate-side plate 5 and a coolant-sideplate 6, respectively. Spacers 8 a, 8 b, 8 a′, and 8 b′ (e.g.,preferably made of the same material as the isolation plates) areprovided to separate the filament rings and to provide strength to thestructure so that it is capable of withstanding a vacuum. The first andsecond plates, however, should not be so thick as to limit the heatingability of the lamps. Outer ring 8 c encloses and provides structuralintegrity to the lamp assemblies (2 a and 2 b). “Hairpins” 9 (FIG. 2)provide support for the filament rings 7 to prevent the filament rings 7from touching the isolation plates (5 and 6). Such hairpins may be madeof different metals or different glasses. In one embodiment, a flattungsten ring element uses small diameter tungsten wire itself asconical support springs equally spaced about the periphery of the heatercoil. In an alternate embodiment, a wavy tungsten filament includes aconical tungsten wire spring/coil which encircles the heating element asis done for support in conventional lamp filaments. The outer circulardiameter fits within the quartz spacing and the inner circular diameterfits around the filament diameter. Small tungsten tripods may beemployed. Any material compatible with the high temperature environmentand quartz tungsten/halogen materials, and any configuration capable ofsupporting the heating element and minimizing contact between theheating element and the quartz encasement is acceptable. Others includealumina or molybdenum posts.

[0023] A shield 10 (e.g., an RF-, a heat- or a combinationRF/heat-shield) is fitted between the heated lamp assemblies (2 a and 2b) and corresponding cooled induction coil assemblies 20 a and 20 b. Thecooled RF induction coils 11 are mounted in a cooled structure 12 (e.g.,a water- or air-cooled cup) (preferably made of a conductive material(e.g., OFHC copper, plated with gold)). In one embodiment, the shield 10is made of a piece of metal plate (e.g., a gold plate) with a radialpattern of slots 10 a therein. In a second embodiment, an isolationplate (e.g., a quartz plate) is coated with a metal (e.g., gold-platedon the side towards the lamp) and formed with a radial pattern of slots;hence comprising the shield 10. The (radially) slotted shield 10 servestwo primary functions, namely, (1) the radial pattern of slots in theshield 10 will permit the RF field to penetrate so that it willinductively couple power with the tungsten rings 7, while (2) theRF/heat shield 10 will keep excessive heat from the lamp assemblies 2from reaching the RF induction coils 11 or the cooling structure 12 bysimply reducing the area and view factors through which the tungstenheating rings 7 can radiate to the cooled inductive coil assembly 20.Moreover, the shield 10 reflects radiant energy to the substrate 3.Measurement openings 13 are provided in the filament ring 7 c, shield 10and cooling structure 12 to enable sensors (e.g., a temperature sensor14) to monitor the condition (e.g., temperature) of the substrate 3.Additional measurement openings 13 may be provided to monitor thecondition (e.g., temperature) of the substrate 3 at additional points.Preferably, the cooling structure 12 includes electrical control signalsthat can be programmed for temperature control and uniformity.

[0024] The cavities 15 in the lamp assemblies 2 may be filled with ananti-darkening agent (e.g., iodine, bromine or any halogen gas for usewith tungsten) to minimize the darkening of the lamp assemblies 2 due tothe evaporation of filament material (e.g., tungsten) from the filament7 coating the inside of the plates 5. As a result, the filament surfaceslast longer through a self-compensating process. The thinner thematerial the hotter locally a continuous current carrying material getsand the more rapid the deposition of filament material; therebyrepairing thin spots.

[0025]FIG. 2 shows a top view of the lamp assembly 2 a including thefilament rings 7 a, 7 b, and 7 c (separated by quartz spacers 8 b and 8c) and a measurement opening 13. Such an opening need not be centeredand is preferably located to optimze the monitoring of the substrate 3temperature, for example as indicated by 13 a and 13 b.

[0026] The isolation plates 5 and 6 and the quartz separators 8 a, 8 b,and 8 c are preferably fused together by fusion bonding. That is, abonding layer (e.g., glass) with a suitable melting point is placedbetween all the connecting surfaces and then a sufficient amount of heatis applied to the layers to fuse them together. The filament rings 7 andthe anti-darkening agent are preferably sealed inside the circularcavity spaces 15 at the same time. Alternatively, “tip offs” may beprovided for evacuating the cavities 15, purging and cycle purging thecavities and then backfilling them with the anti-darkening agent.

[0027] The cavities 15 can be created in many ways, including but notlimited to: (a) by fusing together an upper quartz plate, a lower quartzplate and several nested quartz ring separators; and (b) by fusingtogether an upper flat quartz plate and a lower quartz plate withmachined multiple groove cavities on it. Moreover, the method of fusionis preferably performed under an atmosphere that contains the gas to befilled. For example, the fusing process is done inside a chamber thatcan be heated to fuse the system together while having the gasespresent.

[0028] The substrate-side plate 5 creates the structure for the RTP lamp2 and acts as a vacuum window. Since the substrate-side plate 5 is maderigid and is sealed, it may act as its own vacuum window. This has anadded advantage—because no additional vacuum window has to be added onthe top of this lamp for it to be used in the semiconductor process, theradiation intensity from the heater source to the wafer is not reduced.The ribs of the spacers 8 also minimize the potentially large stresseson the structure. In general, the window preferably is flat, able towithstand the vacuum condition, and support a high radiant heat flux.Stresses sufficient to break the window may occur if the window is madetoo thick (enough to withstand the vacuum) and accordingly absorbs toomuch heat. Such a thickness results in a large thermal gradient throughthe window, and the gradient creates the stresses.

[0029] The coolant-side plate 6 provides the structure for the RTP lamp2 and acts as a heat sink. This is accomplished by cooling the isolationplate either actively or passively.

[0030] In a preferred embodiment, the filament rings 7 are notphysically connected to any power supply or other filament rings. Thisreduces input/output failure due to metal to metal interface fatiguethat commonly occurs in all the tungsten lamps. Preferably, the power ineach of the rings 7 is delivered by induction using an inductivelycoupled power system. As shown in FIG. 3, this inductively coupled powersystem includes a shield 10 and an array of induction coils 11 placedopposite to the filament rings 7. The array is preferably placed on theoutside of the quartz lamp assembly 2. Such induction coils 11 are thendriven by a switching-mode RF power supply. By supplying RF power to theinduction coils 11, inductive currents are generated individually andselectively in each of the filament rings 7 through those radial slots10 a in the shield 10.

[0031] In the preferred embodiment, the lamp 2 includes three separatecircular zones that are separated by conductive boundaries. Power may beapplied to each zone of the lamp 2 separately to maintain a uniformradiation field for heating the wafer. However, alternate numbers ofzones and zone configurations are also possible according to the presentinvention. By selectively and independently controlling the power to thezones of lamps, the center-to-edge power distribution radiated from thelamps can be adjusted over time. If, in response to measurement of thewafer, there is a need to make the center and the edge differenttemperatures, this may be accomplished by adjusting the power input toeach zone of lamps.

[0032] The cooling structure 12 is placed on the backside of the coolantside plate 6. The cooling structure 12 of FIG. 1 includes threegrooves—one for each pair of induction coils 11 and filament rings 7—andpreferably a measurement opening 13. The cooling structure 12 isolatesthe induction coils 11 to substantially reduce leakage of the inducedfields from one coil to any of the filament rings 7 except for the oneto which it is aligned. This structure is essentially like amini-transformer with three primaries, each of these three primaries isinside its own Faraday cage. Each of these primaries will only couplepower to its corresponding filament ring, which act as the secondariesof the transformer and the Faraday cage walls. 11. Each coil antenna 11may be vertically displaced as shown in FIGS. 1 and 3, or they may allreside in the same horizontal plane.

[0033]FIG. 5 presents a second embodiment for inductively coupling powerto the filaments 7. It includes a rolled strip coil powered from aswitching power supply. The AC power supply and coil antenna 11 shouldbe designed for maximum power transfer coupling, i.e. maximum powercoupling efficiency. The AC frequency or band of frequencies isinconsequential. In other words, a crude AC power source (crude relativeto those required for RF power transfer in plasma processing devices)can be employed.

[0034] Alternate embodiments of heaters according to the presentinvention include (1) honeycombs of halogen lamps and (2) straight-linelamp arrays. By powering each lamp separately according to the presentinvention, the substrate to be processed can be heated more uniformly.Similarly, in yet another alternate embodiment, an external structure orplate (e.g., an external carbon structure) is inductively heated andthen re-radiates heat to the wafer.

[0035] In fabricating a quartz-plate-based embodiment of the presentinvention, it is important to properly seal the plates together. If lowmelting point glass is used for bonding the quartz plates together, thedifferent coefficients of thermal expansion between the bonds and theplates may cause stress fractures and/or warping.

[0036] In an alternate embodiment, the quartz plates areelectrostatically bonded at very high temperatures. That is, with allthe elements (e.g., the upper and lower quartz plates, the middle quartzseparator rings, the filament rings inside cavity and the gases in thecavity) in place, a very high voltage is applied on the outside upperand lower quartz plates. All the plates are thus attractedtogether—tightly loading them. A diffusion bond is then made by meltinga layer of the quartz at the interfaces at very high temperature to jointhe layers together. The diffusion bonding preferably uses interfacesthat are free of debris, precisely aligned and in intimate contact.

[0037] In an alternate embodiment of the present invention, the lampsystem is made as a decoupled system (e.g., using a wire sealing of thequartz plates at the outer edges). For example, a thick piece of quartzcan be used to cover over and seal the lamp around the outside edge,followed by a wire seal to another piece that has the cavity in it.Attention should be given to cold areas that tend to condense out theanti-darkening agent and cool part of the filament rings, therebystopping the halogen process. (In order for the halogen process tooperate properly, the whole system has to be hot, around 800 C,including the wire connection points.)

[0038] In yet a further embodiment of the present invention, the RTPlamp is made as a detachable system of several separate parts (e.g., anupper flat quartz plate, a lower quartz plate with machined multiplecircular groove cavities on it, filament rings, and support for thefilament rings). Such a lamp can be disassembled after use or when thefilament burns out. One such detachable system includes clamps/fasteners(e.g., stainless steel clamps with alumina faces) bolting together thelayers that are sealed by a wire seal to hold the vacuum. At least oneheater heats the outside edge to keep the wire seal warm. The lamp thencan be turned on and off as usual. One advantage of such a configurationis that if the filament rings burn out or other elements are damaged,the lamp can be repaired quickly. The lamp is simply taken apart, theelement is replaced, and the lamp is reassembled and resealed. This canbe done as many times as needed as long as the halogen process isrestarted again.

[0039] As would be appreciated by one of ordinary skill in the art, alamp according to the present invention preferably delivers to a wafer apower density having a high watts per square inch. Moreover, a largeview factor lamp system may be placed very close to the wafer. As theview factor increases, the amount of the power needed dropsdramatically. Accordingly, the zoning on the wafer becomes increasinglyadvantageous as the lamp is placed closer to the wafer. Generally, theview factor area of the filament at the wafer can be made arbitrarilylarge. As shown in FIG. 4, the view factor area is the area projected onthe wafer by the solid angle ∝ of the filament. In another words, theview factor area is equal to the solid angle ∝ of the filament 7multiplied by the distance, d, between the filament 7 and the wafer 3.The solid angle ∝ of the filament is the solid angle subtended by thefilament when viewed from the wafer surface, which is determined by thedimension of the tungsten rings 7. By varying the design dimension ofthe filament rings 7, the increasingly large view factor helps togenerate a uniform radiation field for heating the wafer.

[0040] In order to produce a properly tested filament ring (or coil),the present invention preferably utilizes an electronic test apparatusto confirm that the ring (or coil) will inductively receive RF energy.Such an apparatus is preferable to a DC test apparatus for the filamentsince the filament is not operated using DC power.

[0041] As a further testing mechanism, the present invention preferablyincludes a data measurement and storage system for relating applied RFpower to output factors (e.g., heat generated, lamp startup). Such datacan be used to inform a system operator if the lamp is operating out ofits intended range.

[0042] In yet another alternate embodiment, the substrate to beprocessed is moveable while being heated in order to provide a changingand/or more uniform radiant heat flux across the substrate. Thesubstrate may be rotated or translated (either vertically orhorizontally).

[0043] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. In a thermal processing environment for heating a substrate, theimprovement comprising: a first inductively coupled filament for heatingthe substrate.
 2. In the thermal processing environment as claimed inclaim 1, wherein the first inductively coupled filament comprises aninductively coupled halogen lamp filament.
 3. In the thermal processingenvironment as claimed in claim 2, further comprising a reflectiveshield for reflecting light towards the substrate.
 4. In the thermalprocessing environment as claimed in claim 1, wherein the substratecomprises a semiconductor wafer.
 5. In the thermal processingenvironment as claimed in claim 1, wherein the substrate comprises aliquid crystal display panel.
 6. In the thermal processing environmentas claimed in claim 1, further comprising a second inductively coupledfilament.
 7. In the thermal processing environment as claimed in claim6, further comprising plural AC power sources for selectively poweringthe first and second inductively coupled filaments separately.
 8. In thethermal processing environment as claimed in claim 7, further comprisingan outer metal enclosure and a Faraday shield for blocking energy fromone of the plural AC power sources from being coupled to the firstinductively coupled filament while allowing the energy from the one ofthe plural AC power sources to be coupled to the second inductivelycoupled filament.
 9. In the thermal processing environment as claimed inclaim 1, further comprising a first cavity, between first and secondisolation plates, for receiving the first inductively coupled filament.10. In the thermal processing environment as claimed in claim 9, furthercomprising hairpins for supporting the first inductively coupledfilament away from the first and second isolation plates.
 11. In thethermal processing environment as claimed in claim 10, wherein theisolation plates comprise quartz plates.
 12. In the thermal processingenvironment as claimed in claim 10, wherein the isolation platescomprise frit bonded quartz plates.
 13. In the thermal processingenvironment as claimed in claim 10, wherein at least one of theisolation plates comprises a machined plate forming at least a portionof the first cavity.
 14. In the thermal processing environment asclaimed in claim 10, wherein the cavity comprises an anti-darkeningagent.
 15. In the thermal processing environment as claimed in claim 14,wherein the anti-darkening agent comprises iodine.
 16. In the thermalprocessing environment as claimed in claim 14, wherein theanti-darkening agent comprises bromine.
 17. In the thermal processingenvironment as claimed in claim 14, wherein the anti-darkening agentcomprises a halogen gas.
 18. In the thermal processing environment asclaimed in claim 1, wherein the first inductively coupled filamentcomprises a tungsten filament.
 19. In the thermal processing environmentas claimed in claim 1, wherein the first inductively coupled filamentcomprises a tungsten alloy filament.
 20. In the thermal processingenvironment as claimed in claim 10, further comprising a clamp forclamping together the isolation plates.